A balanced magnetic door security switch is typically constructed using one or more reed switches or relays arranged within the switch housing or casing. These reed relays respond only to a permanent magnet moved, or otherwise placed near, to the reed contacts of the relay within a tolerance band in the horizontal, vertical and radial planes. As is well known, and is described in more detail in one or more of the patents discussed below, in a common, normally open (NO) implementation of such a relay switch or relay, the reed contacts or reeds of the reed relay are normally spaced apart in physically overlapping relation and when a moveable permanent magnet is placed in correct relation thereto, the reeds are pulled together by the north-south magnetic flux induced into the reeds. With the reeds on contact, i.e., in the closed state of the switch or relay, current will flow therethrough, thereby completing the circuit in which the switch is located. The size of the switch assembly is determined by the amount of current required to flow through the switch and, for security door applications, the current requirement for driving the input of the system computer is very small and thus the reed switch assembly can be small in size (e.g., as small as 1/2 inch long and 1/8 inch in diameter).
In another common, normally closed (NC) configuration, a small permanent magnet is placed near to the reed switch or relay which biases or drives the reeds into the closed state. When a further, moveable permanent magnet is placed in correct relation with respect to reed switch and fixed permanent magnet, the flux of the moveable magnet will cancel that of the fixed magnet and the reeds will be driven to the open state thereof.
A commercial security door switch system uses combinations of such switches to form a complex electrical circuit. The circuit provides a narrow positioning tolerance band for the moveable magnet assembly, with the tolerance being typically 1/8 to 1 inch, depending on the plane and movement requirements.
Referring now to the patented prior art, U.S. Pat. No. 4,210,889 (Holce) discloses a door switch unit similar to those used commercially in the security industry today, and reference is made to that patent for a more complete description of such door switch units and associated security systems. U.S. Pat. No. 4,908,604 (Jacob) discloses a remotely controlled security system including a test mode wherein each of a plurality of entrance monitors is checked by individually opening each door or window being monitored, which results in the production of a characteristic beep. U.S. Pat. No. 3,641,552 (Friberg) discloses a centrally located access alarm security system including a door sensor device and employing a test switch which, when open, provides triggering of each channel for simultaneous testing of the associated circuitry, indication lamps and audible alarm.
Other patents of possible interest include U.S. Pat. No. 4,365,196 (Finch); U.S. Pat. No. 3,408,493 (Westover et al); U.S. Pat. No. 4,064,452 Toth); U.S. Pat. No. 4,866,377 (Macovschi); and U.S. Pat. N (Mullen). The Finch patent discloses a magnetic proximity sensing transducer for railway crossing signalling installations or for sensing articles on a conveyor line. The transducer is responsive to changes in a magnetic field produced thereby caused by the proximity of an article and, to enable field checking, a field disturbing means is provided for creating an additional field which simulates the disturbance of the magnetic field that would be caused by the proximity of the article. The Westover et al patent discloses a transducer interrogator for a railway wheel transducer which continuously monitors the operation of a magnetic circuit using a saturable reactor located in the magnetic field of a wheel trip. The Toth patent discloses an eddy current defect simulator which controllably alters an electromagnetic field near the detector probe of an eddy current inspection system. The Macovschi patent discloses a proximity detector wherein checking of the functioning thereof is effected by alternately starting and stopping a detector oscillator. The Mullen patent discloses a magnetic detection apparatus using magnetic switches, and static and varying bias fields.